1. Field of the Invention
The present invention relates to a monolithic ceramic capacitor.
2. Description of the Prior Art
Monolithic ceramic capacitors generally comprise a plurality of stacked dielectric ceramic layers, a plurality of internal electrodes formed between two adjacent dielectric ceramic layers, and external electrodes formed on opposite sides of the stacked ceramic layers and connected to the internal electrodes.
Stronium titanate and a small amount of bismuth incorporated wherein have previously been used as dielectric materials because of their relatively large dielectric constant, small voltage dependency of dielectric constant and small dielectric loss. Typical dielectric ceramics of the prior art include a composition of a SrTiO.sub.3 -PbTiO.sub.3 -Bi.sub.2 O.sub.3.sup..multidot. nTiO.sub.2 system (Japanese patent Laid-open No. 49-30900), a dielectric ceramic composition of a SrTiO.sub.3 - MgTiO.sub.3 - Bi.sub.2 O.sub.3 - TiO.sub.2 - Pb.sub.3 O.sub.4 system (Japanese patent publication No. 59-8923), a dielectric ceramic composition of a SrTiO.sub.3 -CaTiO.sub.3 - Bi.sub.2 O.sub.3.sup..multidot. nTiO.sub.2 system (Laid-open No. 59-20908), and a If such compositions are used in monolithic 60-145951).
If such a composition is applied to monolithic ceramic capacitors, the internal electrodes is should be made of material such as platinum and silver-palladium alloys which have high melting points and high resistance to oxidation at high temperatures. This is because the compositions have high sintering temperatures of not less than 1120 .degree. C. A drawback to using platinum and silver-palladium alloy is their high cost. In addition, if any silver-palladium alloy is used as the internal electrode material, it causes migration of silver into the ceramic layers, resulting in lowering the electrical properties of the capacitors. Further, the silver-palladium alloy causes increased equivalent series resistance of the capacitors because of its low conductivity.
To solve these problems, use of copper or a copper alloy has been proposed as a material for internal electrodes because these materials are low in price and high in conductivity. In order to use copper or a copper alloy as an internal electrode material, it is required to fire the dielectric ceramic composition in a reducing atmosphere since such a material has a low melting point and is easily oxidized at the sintering temperature of the dielectric ceramic compositions of the prior art. However, if the dielectric ceramic compositions of the prior art are fired in reducing atmospheres, the bismuth oxide contained therein is reduced during firing, resulting in lowering of the insulating resistance. Thus, it is impossible to use copper or a copper alloy as a material for internal electrodes. SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide an expensive monolithic ceramic capacitor which does not experience decrease in electrical properties as a result of.
According to the present invention, there is provided a monolithic ceramic capacitor including a plurality of dielectric ceramic layers united into one body, a plurality of internal electrodes formed between two adjacent dielectric ceramic layers, and external electrodes formed on opposite sides of the united ceramic layers, each being connected to alternate internal electrodes. The dielectric ceramic layers mainly consists of strontium titanate containing bismuth oxide and an antireducing agent which prevents the composition from reduction. The internal electrodes consist essentially of a copper or a copper alloy.
In a preferred embodiment, the internal electrodes may be incorporated with at least one additive selected from the group consisting of glass frit, dielectric powder, and an antireducing agent. The incorporation of such an additive into the internal electrodes prevents the monolithic ceramic capacitors from delamination between adjacent dielectric layers. The sum of the content of additives in the internal electrodes should be not more than 40 wt%.
It is preferred to use a dielectric ceramic composition of a system, SrTiO.sub.3 -PbTiO.sub.3 -CaTiO.sub.3 -Bi.sub.2 O.sub.3 -SnO.sub.2 -TiO.sub.2, or of a system, SrTiO.sub.3 - MgTiO.sub.3 - Bi.sub.2 O.sub.3 - TiO.sub.2 - Pb.sub.3 O.sub.4 for the dielectric layers of the monolithic ceramic capacitor. Preferably, the dielectric layers are made up of a dielectric ceramic composition expressed by the general formula: EQU .alpha.(Sr.sub.l-x-y-z Pb.sub.x Ca.sub.y Mg.sub.z)TiO.sub.3 +.beta.{Bi.sub.2 O.sub.3.n (Ti.sub.l-m Sn.sub.m) O.sub.2}
wherein x, y, z, .alpha., .beta. and m are mole fractions of the respective components or ingredients, each taking a value within the following respective ranges: .alpha.+.beta.=1.000, 0&lt;.beta..ltoreq.0.300, 0.000 .ltoreq..times..ltoreq.0.500, 0.000 .ltoreq.y.ltoreq.0.500, 0.000.ltoreq.z .ltoreq.0.650, 0.000.ltoreq..times.+y+z.ltoreq.0.750, 0.000.ltoreq.m.ltoreq.0.995, and wherein n takes a value in moles within the following range: 0.00&lt;n.ltoreq.5.00.
To prevent the dielectric ceramic composition from reduction during firing, an antireducing agent may be used therein having a composition expressed by the general formula: EQU .alpha.MnO.sub.2 +.beta.RO+.gamma.B.sub.2 O.sub.3 +(1-.alpha.-.beta.-.gamma.)SiO.sub.2, or EQU .alpha.Li.sub.2 O+.beta.RO+.gamma.B.sub.2 O.sub.3 +(1-.alpha.-.beta.-.gamma.)SiO.sub.2, or EQU .alpha.ZnO+.beta.RO+.gamma.B.sub.2 O.sub.3 +(1-.alpha.-.beta.-.gamma.)SiO.sub.2,
wherein RO is at least one oxide selected from the group consisting of MgO, CaO, SrO and BaO, and wherein .alpha., .beta.and .gamma.are molar percentages of the respective components and take a value within the following respective ranges, 5.ltoreq..alpha..ltoreq.20, 10.ltoreq..beta..ltoreq.60, 20.ltoreq..gamma..ltoreq.40. Copper and copper alloys may be used as a material for internal electrodes;
The incorporation of an antireducing agent into the above basic composition lowers its sintering temperature and prevents it from reduction during firing in a reducing atmosphere. This makes it possible to use copper or a copper alloy as a material for the internal electrodes. In addition, the use of copper or an copper alloy makes it possible to prevent the dielectric layers from migration of the internal electrode material, as well as to reduce the cost of producing the monolithic ceramic capacitors.
The above and other objects, features and advantages of the present invention will become apparent from the following description, taken in connection with the accompanying drawings which show, by way of example only, preferred embodiments thereof.